D5270: tests: sniff for libfuzzer actually being available in test-fuzz-targets.t

[Yuya Nishihara]

> +  $ havefuzz() {
> +  >     cat > $TESTTMP/dummy.cc <<EOF
> +  > #include <stdlib.h>
> +  > int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) { return 0; }

Still stdint.h is missing.